Hypergolic propellant

The hypergolic fuel hydrazine being loaded onto the MESSENGER space probe. Note the safety suit the technician is wearing.

A hypergolicrocket propellant combination used in a rocket engine is one where the propellants spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. Although commonly used hypergolic propellants are difficult to handle because of their extreme toxicity and/or corrosiveness, they can be stored as liquids at room temperature and hypergolic engines are easy to ignite reliably and repeatedly.

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Soviet rocket engine researcher Valentin Glushko experimented with hypergolic fuel as early as 1931. It was initially used for "chemical ignition" of engines, starting kerosene/nitric acid engines with an initial charge of phosphorus dissolved in carbon disulfide.

Starting in 1935, Prof. O. Lutz of the German Aeronautical Institute experimented with over 1000 self-igniting propellants. He assisted the Walter Company with the development of C-Stoff which ignited with concentrated hydrogen peroxide. BMW developed engines burning a hypergolic mix of nitric acid with various combinations of amines, xylidines and anilines.[1]

Hypergolic propellants were discovered independently, for the third time, in the US by GALCIT and Navy Annapolis researchers in 1940. They developed engines powered by aniline and nitric acid.[2]Robert Goddard, Reaction Motors and Curtiss-Wright worked on aniline/nitric acid engines in the early 1940s, for small missiles and jet assisted take-off (JATO).[3]

In Germany from the mid-1930s through World War II, rocket propellants were broadly classed as monergols, hypergols, non-hypergols and lithergols. The ending ergol is a combination of Greekergon or work, and Latin oleum or oil, later influenced by the chemical suffix -ol from alcohol.[Note 1] Monergols were monopropellants, while non-hypergols were bipropellants which required external ignition, and lithergols were solid/liquid hybrids. Hypergolic propellants (or at least hypergolic ignition) were far less prone to hard starts than electric or pyrotechnic ignition. The "hypergole" terminology was coined by Dr. Wolfgang Nöggerath, at the Technical University of Brunswick, Germany.[4] The only rocket-powered fighter ever deployed, the Messerschmitt Me 163B Komet, depended on its methanol/hydrazine fueled, high test peroxide consuming HWK 109-509A rocket motor, using its hypergolic propellants for its fast climb and quick-hitting tactics, at the cost of having a supremely volatile power system capable of causing a massive explosion, with any degree of inattention at any time. Other proposed combat rocket fighters like the Heinkel Julia and reconnaissance aircraft like the DFS 228 were meant to use the Walter 509 series of rocket motors, but besides the Me 163, only the Bachem Ba 349Natter vertical launch expendable fighter was ever flight-tested with the Walter rocket propulsion system as its primary sustaining thrust system for military-purpose aircraft.

The trend among western space launch agencies is away from large hypergolic rocket engines and toward hydrogen/oxygen engines with higher performance. Ariane 1 through 4, with their hypergolic first and second stages (and optional hypergolic boosters on the Ariane 3 and 4) have been retired and replaced with the Ariane 5, which uses a first stage fueled by liquid hydrogen and liquid oxygen. The Titan II, III and IV, with their hypergolic first and second stages, have also been retired. Hypergolic rockets are still widely used in upper stages when multiple burn-coast periods are required.[citation needed]

Hypergolic rockets are usually simple and reliable because they need no ignition system. Although larger hypergolic engines in some launch vehicles use turbopumps, most hypergolic engines are pressure fed. A gas, usually helium, is fed to the propellant tanks under pressure through a series of check and safety valves. The propellants in turn flow through control valves into the combustion chamber; there, their instant contact ignition prevents a mixture of unreacted propellants from accumulating and then igniting in a potentially catastrophic hard start.

Because hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted and are therefore uniquely suited for spacecraft maneuvering and well suited, though not uniquely so, as upper stages of such space launchers as the Delta II and Ariane 5, which must perform more than one burn. Restartable cryogenic (oxygen/hydrogen) rocket engines nevertheless exist, notably the RL-10 on the Centaur and the J-2 on the Saturn V.

Relative to their mass, traditional hypergolic propellants are less energetic than such cryogenic propellant combinations as liquid hydrogen / liquid oxygen or liquid methane / liquid oxygen. A launch vehicle that uses hypergolic propellant must therefore carry a greater mass of fuel than one that uses these cryogenic fuels.

Kerosene + (high-test peroxide + catalyst) – Gamma, with the peroxide first decomposed by a catalyst. Cold hydrogen peroxide and kerosene are not hypergolic, but concentrated hydrogen peroxide (referred to as high-test peroxide or HTP) run over a catalyst produces free oxygen and steam at over 700 °C (1,300 °F) which is hypergolic with kerosene.[12]